A power metal-oxide-semiconductor field-effect transistor (MOSFET) is a type of transistor that is adapted for use in high power applications. Generally, a power MOSFET device has a vertical structure, wherein a source and gate contact are located on a first surface of the MOSFET device that is separated from a drain contact by a drift layer formed on a substrate. Vertical MOSFETS are sometimes referred to as vertical diffused MOSFETs (VDMOSFETs) or double-diffused MOSFETs (DMOSFETs). Due to their vertical structure, the voltage rating of a power MOSFET is a function of the doping and thickness of the drift layer. Accordingly, high voltage power MOSFETs may be achieved with a relatively small footprint.
FIG. 1 shows a conventional power MOSFET device 10. The conventional power MOSFET device 10 includes a substrate 12, a drift layer 14 formed over the substrate 12, one or more junction implants 16 in the surface of the drift layer 14 opposite the substrate, and a junction gate field effect transistor (JFET) region 18 between each one of the junction implants 16. Each one of the junction implants 16 is formed by an ion implantation process, and includes a deep well region 20, a base region 22, and a source region 24. Each deep well region 20 extends from a corner of the drift layer 14 opposite the substrate 12 downwards towards the substrate 12 and inwards towards the center of the drift layer 14. The deep well region 20 may be formed uniformly or include one or more protruding regions, as shown in FIG. 1. Each base region 22 is formed vertically from the surface of the drift layer 14 opposite the substrate 12 down towards the substrate 12 along a portion of the inner edge of each one of the deep well regions 20. Each source region 24 is formed in a shallow portion on the surface of the drift layer 14 opposite the substrate 12, and extends laterally to overlap a portion of the deep well region 20 and the base region 22, without extending over either. The JFET region 18 defines a channel width 26 between each one of the junction implants 16.
A gate oxide layer 28 is positioned on the surface of the drift layer 14 opposite the substrate 12, and extends laterally between a portion of the surface of each source region 24, such that the gate oxide layer 28 partially overlaps and runs between the surface of each source region 24 in the junction implants 16. A gate contact 30 is positioned on top of the gate oxide layer 28. Two source contacts 32 are each positioned on the surface of the drift layer 14 opposite the substrate 12 such that each one of the source contacts 32 partially overlaps both the source region 24 and the deep well region 20 of one of the junction implants 16, respectively, and does not contact the gate oxide layer 28 or the gate contact 30. A drain contact 34 is located on the surface of the substrate 12 opposite the drift layer 14.
In operation, when a biasing voltage is not applied to the gate contact 30 and the drain contact 34 is positively biased, a junction between each deep well region 20 and the drift layer 14 is reverse biased, thereby placing the conventional power MOSFET 10 in an OFF state. In the OFF state of the conventional power MOSFET 10, any voltage between the source and drain contact is supported by the drift layer 14. Due to the vertical structure of the conventional power MOSFET 10, large voltages may be placed between the source contacts 32 and the drain contact 34 without damaging the device.
FIG. 2 shows operation of the conventional power MOSFET 10 when the device is in an ON state. When a positive bias is applied to the gate contact 30 of the conventional power MOSFET 10, an inversion layer channel 36 is formed at the surface of the drift layer 14 underneath the gate contact 30, thereby placing the conventional power MOSFET 10 in an ON state. In the ON state of the conventional power MOSFET 10, current (shown by the shaded region in FIG. 2) is allowed to flow from each one of the source contacts 32 through the inversion layer channel 36 and into the JFET region 18 of the drift layer 14. Once in the JFET region 18, current flows downward through the drift layer 14 towards the drain contact 34. An electric field presented by junctions formed between the deep well region 20, the base region 22, and the drift layer 14 constricts current flow in the JFET region 18 into a JFET channel 38 having a JFET channel width 40. At a certain spreading distance 42 from the inversion layer channel 36 when the electric field presented by the junction implants 16 is diminished, the flow of current is distributed laterally, or spread out in the drift layer 14, as shown in FIG. 2. The JFET channel width 40 and the spreading distance 42 determine the internal resistance of the power MOSFET 10, thereby dictating the performance of the device. A conventional power MOSFET 10 generally requires a channel width 26 of 3 microns or wider in order to sustain an adequate JFET channel width and 40 spreading distance 42 for proper operation of the device.
The electric field formed by the junctions between the deep well region 20, the base region 22, and the drift layer 14 radiates through the gate oxide layer 28, thereby physically degrading the gate oxide layer 28 over time. Eventually, the electric field will cause the gate oxide layer 28 to break down, and the conventional power MOSFET 10 will cease to function.
Accordingly, a power MOSFET is needed that is capable of handling high voltages in the OFF state while maintaining a low ON state resistance and having an improved longevity.